Here's how to determine this:

* Lithium (Li) is in group 1 of the periodic table, meaning it forms a +1 cation (Li⁺).

* Phosphorus (P) is in group 15 of the periodic table, meaning it forms a -3 anion (P³⁻).

* To balance the charges, you need three lithium ions (3 x +1 = +3) to combine with one phosphide ion (-3).

This gives you the formula Li₃P.